A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create a signal with a very precise frequency.
This frequency is commonly used to keep track of time, to provide a stable clock signal required by a digital system, and/or to stabilize frequencies for radio transmitters and receivers.
The most common type of piezoelectric material used in crystal oscillators is quartz crystal, but other materials like polycrystalline ceramics are also used.
Typically, quartz crystals are cut and mounted to vibrate best at a desired resonance frequency or a multiple of the desired resonant frequency. When the crystal is vibrating, it can be modeled as an RLC circuit producing a rapidly changing reactance with frequency, wherein the RLC circuit provides positive feedback and gain at the resonant frequency therefore producing sustained oscillations.
FIG. 1 illustrates a current-controlled CMOS-inverter oscillator circuit as known from E. Vittoz, “Low-Power Crystal and MEMS Oscillators: The Experience of Watch Developments”, Integrated Circuits and Systems, FIG. 5.25, page 129, DOI 10.1007/978-90-481-9394-3. The circuit of FIG. 1 comprises a transistor T1 having a gate G1, a source S1, and a drain D1, a transistor T2 having a gate G2, a source S2, and a drain D2, a transistor T3 having a gate G3, a source S3, and a drain D3, a capacitor C1 having a first end 10 and a second end 11, a capacitor C2 having a first end 12 and a second end 13, a capacitor C3 having a first end 14 and a second end 15, a resistor R1 having a first end 16 and a second end 17 and a crystal oscillator 18 having a first end 19 and a second end 20. The source S1 is connected to the source S2 and to the second end 15 of the capacitor C3. The drain D2 is connected to the second end 17 of the resistor R1, to the second end 20 of the crystal oscillator 18, to the drain D3 and to the second end 13 of the capacitor C2. The gate G2 is connected to the first end 16 of the resistor R1, to the first end 19 of the crystal oscillator 18, to the gate G3 and to the second end 11 of the capacitor C1. The first end 10 of the capacitor C1 is connected to the source S3, to the first end 12 of the capacitor C2 and to the first end 14 of the capacitor C3.
In the known circuit according to FIG. 1, the resistor R1 is a feedback resistor from the drains D2 and D3, respectively, of the transistors T2 and T3, respectively, to the gates G2 and G3, of the transistors T2 and T3, respectively, to ensure that the DC voltage levels of these drains D2 and D3 and these gates G2 and G3 of the transistors T2 and T3, are the same. Therefore, the DC voltage level at both terminals 19, 20 of the oscillator crystal 18 is the same. Feedback resistor R1 should have a very high resistance value in case of low power requirements, as it is consuming electrical power all the time.